Fresnel lens sheet and transmission type screen comprising it

ABSTRACT

The present invention provides a Fresnel lens sheet capable of effectively preventing such troubles as the production of double images that occurs due to stray light. A Fresnel lens sheet ( 1 ) comprises a base ( 1   a ) in sheet form, and a plurality of prisms ( 2 ) formed on the incident side of the base ( 1   a ). Each prism ( 2 ) has a plane of refraction ( 3 ) that refracts imaging light (LS) projected from a projector ( 5 ), and a plane of total reflection ( 4 ) that totally reflects, toward the viewer&#39;s side, at least a part of the light that has been refracted at this plane of refraction ( 3 ). At least some of a plurality of the prisms ( 2 ) are so made that the imaging light (LS) that has passed through these prisms ( 2 ) emerges in the direction inclined toward the projector ( 5 ) side at predetermined angles (γb, γc) with respect to the normal perpendicular to the sheet plane of the base ( 1   a ).

TECHNICAL FIELD

The present invention relates to a Fresnel lens sheet for use in a rearprojection type television, particularly to a Fresnel lens sheet usefulfor condensing imaging light obliquely projected from a projector (lightsource) placed at its rear to let the light emerge toward the viewer'sside as nearly parallel rays, and to a rear projection screen comprisingthe Fresnel lens sheet.

BACKGROUND ART

Conventionally known as a large-size-screen television is a rearprojection type television in which imaging light (projected light) isprojected on a rear projection screen from a projector placed at therear of the rear projection screen to display an image to viewers.

In such a rear projection type television, imaging light projected fromthe projector is spread and the spread light is projected on the rearprojection screen, so that it is necessary to keep a certain distancebetween the projector and the rear projection screen. A problem with atelevision of this type, therefore, is that the space in the directionof depth is apt to become large.

In view of this problem, there has been proposed so far a rearprojection type television in which, in order to make the space in thedirection of depth smaller, a projector 5 is placed obliquely below arear projection screen 10′ at the rear of it, as shown in FIG. 13,whereby the projector 5 is allowed to project imaging light LS obliquelyand upwardly on the rear projection screen 10′.

Conveniently used as the rear projection screen 10′ in a rear projectiontype television as is shown in FIG. 13 is a rear projection screencomprising a total reflection Fresnel lens, capable of condensingimaging light projected obliquely from its rear (Japanese Laid-OpenPatent Publication No. 208041/1986). The total reflection Fresnel lensherein denotes a lens having a plurality of prisms, where imaging lightis refracted at the first plane (plane of refraction) of each prism andis then totally reflected at the second plane (plane of totalreflection) of each prism to emerge toward the viewer's side.

In a rear projection screen comprising such a total reflection Fresnellens, the light path of imaging light is adjusted by totally reflectingthe imaging light. Therefore, the rear projection screen can attain hightransmittance even when imaging light is obliquely incident on the rearprojection screen at a large angle.

However, the conventional rear projection screen described above isconfronted with the following problem. Namely, in an area of a Fresnellens sheet 1′ constituting the conventional rear projection screen, onwhich area imaging light LS is incident at a small angle (an area closeto a projector 5), a part of the imaging light LS refracted at the planeof refraction 3′ of each prism 2′ passes through the prism 2′ withoutbeing totally reflected at the plane of total reflection 4′ of the prism2′ and becomes stray light LY, as shown in FIG. 14. The stray light LYthus produced is, as shown in FIG. 15, reflected at the plane ofemergence 1 b′ of the Fresnel lens sheet 1′ and returns to the incidentside via a base 1 a′; this light repeatedly causes incidence andemergence while passing through a plurality of the prisms 2′, andfinally emerges from the plane of emergence 1 b′. As shown in FIG. 15,the stray light LY emerging from the plane of emergence 1 b′ in thismanner comes out from a point that is different from the point fromwhich the imaging light LS is totally reflected from the plane of totalreflection 4′ of the prism 2′, after being refracted at the plane ofrefraction 3′ of the prism 2′, comes out as normal light LZ. The straylight LY, therefore, causes such troubles as the production of doubleimages to decrease image visibility.

SUMMARY OF THE INVENTION

The present invention has been accomplished in the light of theabove-described problem. An object of the present invention is thereforeto provide a Fresnel lens sheet useful for condensing imaging lightobliquely projected from a projector (light source) placed at its rearto let the light emerge toward the viewer's side as nearly parallelrays, capable of reducing the production of stray light that occurs inan area on which imaging light is incident at a small angle, therebyeffectively preventing such troubles as the production of double images;and a rear projection screen comprising the Fresnel lens sheet.

A Fresnel lens sheet according to the present invention, useful forcondensing imaging light obliquely projected from a projector placed atits rear to let the light emerge toward a viewer's side as nearlyparallel rays, comprises a base in sheet form; and a plurality of prismsformed on an incident side of the base, each of the prisms having aplane of refraction that refracts the imaging light projected and aplane of total reflection that totally reflects, toward the viewer'sside, at least a part of the light refracted at this plane ofrefraction, and is characterized in that at least some of the pluralityof prisms are so made that the imaging light that has passed throughthese prisms emerges in a direction inclined toward a projector side ata predetermined angle with respect to a normal perpendicular to a sheetplane of the base.

In the Fresnel lens sheet according to the present invention, it ispreferable that the at least some of the plurality of prisms be thoseprisms that are situated in an area on which the imaging light projectedfrom the projector is incident at a small angle to become stray light.Specifically, it is preferable that the imaging light that has passedthrough the prisms situated in an area on which the imaging light isincident at an angle of 45° or less be inclined toward the projectorside, and it is more preferable that the imaging light that has passedthrough the prisms situated in an area on which the imaging light isincident at an angle of 40° or less be inclined toward the projectorside. This is because stray light tends to be produced in an area onwhich imaging light is incident at angle of 45° or less, particularly40° or less. However, not only the prisms situated in theabove-described area, but also those prisms that are situated in anyarea covering the above-described area (e.g., the entire area of thesheet plane) may be made so that the imaging light that has passedthrough these prisms is inclined toward the projector side.

Further, in the Fresnel lens sheet according to the present invention,it is preferable that the plurality of prisms be so made that the angleat which the imaging light that has passed through the prisms isinclined increases gradually as an angle at which the imaging light isincident on the prisms decreases.

Furthermore, it is preferable that the Fresnel lens sheet according tothe present invention further comprises, at least on a specific part ofan emergent-side surface of the base, from which part the imaging lightemerges in the direction inclined toward the projector side, alow-refraction layer made from a material whose refractive index islower than that of a material for forming the base.

Furthermore, it is preferable that the Fresnel lens sheet according tothe present invention further comprises a light-diffusing elementprovided at least on a specific part of an emergent-side surface of thebase, from which part the imaging light emerges in the directioninclined toward the projector side. In this case, the light-diffusingelement is preferably a rough surface or lenticular lens provided on theemergent side of the base.

Furthermore, it is preferable that the Fresnel lens sheet according tothe present invention further comprises, on an emergent side of thebase, a plurality of additional prisms by which, of the imaging lightthat has passed through the prisms, the imaging light inclined towardthe projector side relative to the normal perpendicular to the sheetplane of the base is adjusted to emerge nearly vertically to the sheetplane of the base.

A rear projection screen according to the present invention comprisesthe above-described Fresnel lens sheet; and a light-diffusing sheetplaced on the viewer's side of the Fresnel lens sheet.

According to the present invention, at least some of a plurality of theprisms formed on the incident side of the base (e.g., those prisms thatare situated in an area on which imaging light projected from aprojector is incident at a small angle to become stray light) are somade that the imaging light that has passed through these prisms emergesin the direction inclined toward the projector side at a predeterminedangle with respect to the normal perpendicular to the sheet plane of thebase, so that the relative positions of the apices of these prisms areshifted toward the side opposite to the projector. For this reason, itis possible to reduce the production of stray light, and is thuspossible to effectively prevent such troubles as the production ofdouble images.

Further, according to the present invention, by making the prisms sothat the angle at which the imaging light that has passed through theprisms is inclined increases gradually as the angle at which the imaginglight is incident on the prisms decreases, it becomes possible to makethe direction in which the imaging light that has passed through theprisms emerges not vary discontinuously and abruptly on the sheet plane.It is, therefore, possible to prevent viewers who are viewing thedisplayed image from feeling that the image is somewhat defective.

Furthermore, according to the present invention, if a low-refractionlayer made from a material whose refractive index is lower than that ofa material for forming the base is provided on a specific part of theemergent-side surface of the base, from which part the imaging lightemerges in the direction inclined toward the projector side, thereflection of the imaging light emerging in the direction inclinedtoward the projector side that occurs at the emergent-side surface ofthe base is reduced. It is, therefore, possible to prevent lowering ofcontrast or the like that is caused by the reflected light.

Furthermore, according to the present invention, by providing alight-diffusing element on a specific part of the emergent-side surfaceof the base, from which part the imaging light emerges in the directioninclined toward the projector side, even if the imaging light emergingin the direction inclined toward the projector side is reflected at theemergent-side surface of the base, the imaging light is reflected withdiffusion. It is, therefore, possible to prevent lowering of contrast orthe like that is caused by the reflected light.

Furthermore, according to the present invention, if a plurality ofadditional prisms are provided on the emergent side of the base,whereby, of the imaging light that has passed through the prisms formedon the incident side of the base, the imaging light inclined toward theprojector side relative to the normal perpendicular to the sheet planeof the base is adjusted to emerge nearly vertically to the sheet planeof the base, it is possible to display a bright image to viewers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a rear projection screen comprisinga Fresnel lens sheet according to the first embodiment of the presentinvention, together with a projection system for the screen (aprojection system that projects imaging light obliquely on the rearprojection screen);

FIGS. 2A, 2B and 2C are sectional views of areas A, B and C of theFresnel lens sheet shown in FIG. 1, respectively, taken along line X-Xin FIG. 1;

FIG. 3A is an illustration showing the behavior of imaging light inthose prisms situated in an area of the Fresnel lens sheet shown in FIG.1, on which area imaging light is incident at a small angle, and FIG. 3Bis an illustration showing the behavior of imaging light in those prismssituated in an area of a conventional Fresnel lens sheet correspondingto the area shown in FIG. 3A;

FIG. 4 is a perspective view showing a modification of the rearprojection screen comprising the Fresnel lens sheet shown in FIG. 1;

FIG. 5 is a sectional view, in the direction of thickness, showing aFresnel lens sheet according to the second embodiment of the presentinvention;

FIG. 6 is a view showing a conventional Fresnel lens sheet forcomparison with the Fresnel lens sheet shown in FIG. 5;

FIG. 7 is a sectional view, in the direction of thickness, showing amodification of the Fresnel lens sheet shown in FIG. 5;

FIG. 8 is a sectional view, in the direction of thickness, showinganother modification of the Fresnel lens sheet shown in FIG. 5;

FIG. 9 is a view showing a rear projection screen comprising the Fresnellens sheet shown in FIG. 5, 7 or 8, along with a projection system forthe screen;

FIG. 10 is a sectional view, in the direction of thickness, showing aFresnel lens sheet according to the third embodiment of the presentinvention;

FIG. 11 is a sectional view, in the direction of thickness, showing amodification of the Fresnel lens sheet shown in FIG. 10;

FIG. 12 is a diagram showing the relationship between the radius of theprism and the angle of inclination of the imaging light in the Fresnellens sheets of Examples 1 to 3;

FIG. 13 is a view showing a conventional rear projection screen togetherwith a projection system for the screen (a projection system thatprojects imaging light obliquely on the rear projection screen);

FIG. 14 is a sectional view, in the direction of thickness, showing aconventional Fresnel lens sheet; and

FIG. 15 is an illustration for explaining how a double image is produceddue to stray light in a conventional Fresnel lens sheet.

EMBODIMENTS OF THE INVENTION

Embodiments of the present invention will be described hereinafter withreference to the accompanying drawings.

First Embodiment

First of all, a Fresnel lens sheet according to the first embodiment ofthe present invention and a rear projection screen comprising it will bedescribed with reference to FIGS. 1 to 4. The Fresnel lens sheetaccording to the first embodiment of the present invention is used as arear projection screen or an optical member of a rear projection screenfor use in a rear projection type television, and comprises a pluralityof prisms (total reflection Fresnel lens) that condense imaging lightobliquely projected from a projector (light source) placed at the rearof the Fresnel lens sheet to let the light emerge toward the viewer'sside as nearly parallel rays. In the first embodiment of the presentinvention, in order to reduce the production of stray light, at leastsome of a plurality of the prisms are so made that the direction inwhich the imaging light that has passed through these prisms emergespartly changes on the sheet plane of the Fresnel lens sheet. Thearrangement of the Fresnel lens sheet and a process of producing theFresnel lens sheet are explained below in detail.

(Arrangement of Fresnel Lens Sheet)

As shown in FIG. 1, the Fresnel lens sheet 1 according to the firstembodiment of the invention is used as a rear projection screen 10 onwhich imaging light LS is projected obliquely from a projector 5, andhas a plurality of prisms 2 in the shape of circular arcs formed on theincident-side surface of a base 1 a in sheet form. A plurality of theprisms 2 are herein disposed concentrically around the point Ppositioned below the center of the lower edge of the Fresnel lens sheet1.

FIGS. 2A, 2B and 2C are sectional views of areas A, B and C of theFresnel lens sheet 1 shown in FIG. 1, respectively, taken along line X-Xin FIG. 1.

As shown in FIGS. 2A to 2C, each prism 2 is formed to have a triangularcross section, and has a plane of refraction 3 that refracts imaginglight LS projected from the projector 5 placed at the rear of theFresnel lens sheet 1 and a plane of total reflection 4 that totallyreflects, toward the viewer's side, at least a part of the light thathas been refracted at this plane of refraction 3.

Conventionally, a plurality of the prisms 2 are so made that the imaginglight LS that has passed through the prisms 2 emerges in the directionof the normal perpendicular to the sheet plane of the base 1 a. In thefirst embodiment of the present invention, however, at least some of theplurality of prisms 2 are so made that the imaging light LS that haspassed through these prisms 2 emerges in the direction inclined towardthe projector 5 side at a predetermined angle γ with respect to thenormal perpendicular to the sheet plane of the base 1 a. Specifically,of a plurality of the prisms 2, at least those prisms that are situatedin an area on which imaging light LS projected from the projector 5 isincident at a small angle θ to become stray light (e.g., an area onwhich imaging light LS is incident at an angle of 45° or less, morepreferably 40° or less) are so made that the imaging light LS that haspassed through these prisms 2 is inclined toward the projector 5 side.However, not only the prisms 2 situated in the above-described area, butalso those prisms 2 that are situated in any area covering theabove-described area (e.g., the entire area of the sheet plane) may beso made that the imaging light LS that has passed through these prisms 2is inclined toward the projector 5 side.

The prisms 2 are herein so made that the inclination, toward theprojector 5 side, of the direction in which the imaging light LS emergesfrom the prisms 2 gets greater as the angle of incidence θ at which theimaging light LS projected from the projector 5 is incident on theprisms 2 decreases.

Specifically, for example, those prisms 2 a that are situated in thearea A distant from the projector 5, on which area imaging light LS isincident at a large angle θ (angle of incidence θ=θa), are so made thatthe imaging light LS emerges from these prisms 2 a in the directionnearly vertically to the sheet plane of the Fresnel lens sheet 1 (seeFIG. 2A). On the contrary, those prisms 2 b that are situated in thearea B on which imaging light LS is incident at an angle θ smaller thanthe angle at which imaging light LS is incident on the area A (angle ofincidence θ=θb<θa) are so made that the imaging light LS emerges fromthese prisms 2 b in the direction inclined toward the projector 5 sideat an angle γb with respect to the normal perpendicular to the sheetplane of the Fresnel lens sheet 1 (see FIG. 2B). Further, those prisms 2c that are situated in the area C on which imaging light LS is incidentat an angle θ smaller than the angles at which imaging light LS isincident on the area A and on the area B (the angle of incidenceθ=θc<θb<θa) are so made that the imaging light LS emerges from theseprisms 2 c in the direction inclined toward the projector 5 side at anangle γc (γc>γb) with respect to the normal perpendicular to the sheetplane of the Fresnel lens sheet 1 (see FIG. 2C). Those prisms 2 that aresituated between the respective prisms 2 (2 a, 2 b, 2 c) shown in FIGS.2A to 2C are so made that the angle γ at which the imaging light LS thathas passed through the prisms 2 is inclined increases gradually as theangle θ at which the imaging light LS is incident on the prisms 2decreases.

In the above-described embodiment, the prisms 2 are so made that thedirection in which the imaging light LS that has passed through theprisms 2 emerges varies gradually on the sheet plane. The prisms 2 mayalso be made so that the direction in which the imaging light LS thathas passed through the prisms 2 emerges is inclined at a fixed angle γon a predetermined part of the sheet plane, or that the direction inwhich the imaging light LS that has passed through the prisms 2 emergesis inclined at a fixed angle γ on the entire part of the sheet plane.

Thus, in the Fresnel lens sheet 1 shown in FIG. 1 and FIGS. 2A to 2C,the shape of each prism 2 is varied depending upon the angle θ at whichimaging light LS is incident on the prism 2, and the prism 2 on whichimaging light LS is incident at a smaller angle θ is so made that theimaging light that has passed through this prism 2 emerges in thedirection inclined more greatly toward the projector 5 side. For thisreason, the following characteristic features are imparted to theFresnel lens sheet 1.

The characteristic features of the Fresnel lens sheet 1 shown in FIG. 1and FIGS. 2A to 2C will be described hereinafter with reference to FIGS.3A and 3B. FIG. 3A is an illustration showing the behavior of imaginglight LS in those prisms 2 c situated in the area C of the Fresnel lenssheet shown in FIG. 1, on which area the imaging light LS is incident ata small angle θ, while FIG. 3B is an illustration showing the behaviorof imaging light LS in those prisms 2 c′ situated in an area of aconventional Fresnel lens sheet 1′ corresponding to the area C. Thefollowing are herein assumed for consideration: the imaging light LSemerges from the prisms 2 c in the direction inclined toward theprojector 5 side at an angle of 10° with the normal perpendicular to thesheet plane of the Fresnel lens sheet 1; the imaging light LS emergesfrom the prisms 2 c′ in the direction of the normal perpendicular to thesheet plane of the Fresnel lens sheet 1′; both the angle at which theimaging light LS is incident on the prisms 2 c and the angle at whichthe imaging light LS is incident on the prisms 2 c′ are 38°; both theapical angle (the angle between the plane of refraction and the plane oftotal reflection) of each prism 2 c and the apical angle of each prism 2c′ are 38°; and both the refractive index of each prism 2 c and that ofeach prism 2 c′ are 1.55.

As shown in FIG. 3B, of the imaging light LS incident on the prisms 2 c′on the conventional Fresnel lens sheet 1′, the light incident on theportion S₁′ is totally reflected at the plane of total reflection 4′ andemerges as normal light LZ, while the light incident on the portion S₂′passes through without being totally reflected at the plane of totalreflection 4′ and becomes stray light LY. At this time, the percentageof the imaging light LS that becomes stray light LY (the light incidenton the portion S₂′) to all of the imaging light LS incident on eachprism 2 c′ (the total of the light incident on the portion S₁′ and thelight incident on the portion S₂′) becomes approximately 25%. This meansthat the stray light LY is produced at a high rate.

On the contrary, as shown in FIG. 3A, the prisms 2 c on the Fresnel lenssheet 1 according to the first embodiment of the present invention areso made that the imaging light LS that has passed through these prisms 2c emerges in the direction inclined toward the projector 5 side. Therelative positions of the apices Q of the prisms 2 c are, therefore,shifted toward the side opposite to the projector 5 (the upper side ofFIG. 3A) as compared with the apices of the prisms 2 c′ on theconventional Fresnel lens sheet 1′ shown in FIG. 3B (indicated by thedotted lines in FIG. 3A). By so making the prisms 2 c on the Fresnellens sheet 1, the percentage of the imaging light LS that becomes straylight LY (the light incident on the portion S₂′) becomes approximately13%, much lower than the above-described percentage, and such troublesas the production of double images is thus prevented.

As described above, the Fresnel lens sheet 1 according to the firstembodiment of the present invention can significantly decrease the straylight LY production rate as compared with the conventional Fresnel lenssheet 1′.

To consider, in more detail, the features of the Fresnel lens sheet 1according to the first embodiment of the present invention, the straylight LY production rates at the prisms on which imaging light LS isincident at angles of 35°, 38° and 40° are shown in Table 1. To obtainthe stray light production rates, the following were assumed: the apicalangle of each prism 2 is 38°; the refractive index of each prism 2 is1.55; and the imaging light LS that has passed through the prisms 2emerges in the direction inclined toward the projector 5 side (downward)at an angle of 3°, 6.5° or 10°. For comparison, the stray light LYproduction rates when the imaging light LS that has passed through theprisms 2 emerges in the direction of the normal perpendicular to thesheet plane of the Fresnel lens sheet 1 are also shown in Table 1.

TABLE 1 Direction of Angle of Angle of Angle of Emergence Incidence 35°Incidence 38° Incidence 40° Perpendicular to lens 30% 25% 18% plane(conventional Fresnel lens sheet) Downward at 3° 24% 18% 14% Downward at6.5° 22% 16% 11% Downward at 10° 20% 13%  8%

As shown in the above Table 1, as compared with the conventional Fresnellens sheet 1′, the Fresnel lens sheet 1 according to the firstembodiment of the present invention shows lower stray light LYproduction rates for the imaging light LS incident on the prisms at anyangle. In the case of the Fresnel lens sheet 1 according to the firstembodiment of the present invention, the stray light LY production ratedecreases as the inclination of the direction in which the imaging lightLS that has passed through the prisms 2 emerges gets greater from 3° to6.5° and to 10° relative to the normal perpendicular to the sheet plane.

However, if the direction in which the imaging light LS that has passedthrough the prisms 2 emerges is excessively inclined toward theprojector 5 side, the image displayed to viewers becomes dark unless ameans for restoring the inclination of the direction in which theimaging light LS emerges is provided, as in Fresnel lens sheets 41 and46 according to the third embodiment of the present invention that willbe described later. It is, therefore, preferable that the direction inwhich the imaging light LS emerges be inclined at an angle of not morethan a certain degree (e.g., not more than 13°) with respect to thenormal perpendicular to the sheet plane.

(Process of Producing Fresnel Lens Sheet)

Next, a process of producing the Fresnel lens sheet 1 according to theaforementioned first embodiment will be described. The explanation willbe hereinafter given with reference to the case where anultraviolet-curing resin method is employed to produce the Fresnel lenssheet 1.

In this case, a mold is firstly prepared by engraving, using a cuttingtool or the like, a mold material such as aluminum, brass or copper withthe shape corresponding to the incident-side surface of the Fresnel lenssheet 1.

Thereafter, an ultraviolet-curing resin is applied to this mold. Acoating method such as roll, gravure, dispenser or die coating may beused for this purpose.

Subsequently, a substantially transparent substrate that transmitsultraviolet light is laid on the ultraviolet-curing resin applied to themold and is pressed by a pressure roller or the like, thereby bringingthe substrate into close contact with the ultraviolet-curing resin. Fromabove the substrate laminated in this manner, ultraviolet light isapplied to cure the ultraviolet-curing resin.

The ultraviolet-curing resin thus cured is then released from the moldand is cut into the desired size, thereby obtaining a Fresnel lens sheet1.

In the case where prisms 2 with excessively small apical angles areformed on the Fresnel lens sheet 1, the cutting tool or the like iseasily broken and cutting becomes difficult while the mold material isengraved by using the cutting tool or the like as mentioned above. Inaddition, in this case, it is difficult to release the molded productfrom the mold. For this reason, if the mass productivity of the Fresnellens sheet 1 is taken into consideration, it is preferable to make theapical angles of the prisms 2 between 35° and 45°.

Further, if the tip angle of the cutting tool or the like is fixed at acertain degree, the efficiency in the production of the mold becomesgreat. Therefore, if the efficiency in the production of the Fresnellens sheet 1 is taken into consideration, it is preferable that theapical angles of all of the prisms 2 be the same.

Although the above explanation has been given with reference to the casewhere the ultraviolet-curing resin method is used as a process ofproducing the Fresnel lens sheet 1, the Fresnel lens sheet 1 can beproduced not only by this ultraviolet-curing resin method but also by(1) a method in which a light-transmitting resin material in the moltenstate is loaded in a mold in the shape corresponding to the Fresnel lenssheet 1 and is cured, and the molded product is then released from themold (casting method); (2) a method in which a light-transmitting resinmaterial that has been heated is loaded in the same mold as in the abovemethod (1) and is molded by the application of pressure, and the moldedproduct is then released from the mold (hot-pressing method); or thelike. The light-transmitting resin material used in the above methodsincludes acrylic, styrene, polycarbonate, and epoxy resins.

(Rear Projection Screen Comprising Fresnel Lens Sheet)

Although the Fresnel lens sheet 1 shown in FIG. 1 constitutes a rearprojection screen 10 by itself, the Fresnel lens sheet 1 may be combinedwith other optical members to produce the rear projection screen 10.

FIG. 4 is a perspective view showing a modification of the rearprojection screen 10 shown in FIG. 1. In the rear projection screen 10shown in FIG. 4, a lenticular lens sheet (light-diffusing sheet) 11 isplaced on the viewer's side of the Fresnel lens sheet 1, and is thusallowed to diffuse, in both the vertical direction (direction of up anddown) and the horizontal direction (direction of right and left), theimaging light condensed by the Fresnel lens sheet 1. On theincident-side surface of the lenticular lens sheet 11, linear lenseswith half-elliptic cross-sections 12 are formed in the verticaldirection (direction of up and down in FIG. 4); light-absorbing layers13 are formed on the surfaces of these lenses with half-ellipticcross-sections 12; and a diffusing agent 14 is dispersed in the lenseswith half-elliptic cross-sections 12. Such a rear projection screen 10as is shown in FIG. 4 may contain, instead of the lenticular lens sheet11, a light-diffusing sheet having any other light-diffusing element,and the same effects can be obtained even in this case.

In the use of the rear projection screen 10 shown in FIG. 1 or 4, aprojector 5 is placed obliquely below the screen 10 at its rear, wherebythe projector 5 is allowed to project imaging light LS obliquely andupwardly on the rear projection screen 10 for displaying an image toviewers.

At this time, owing to the above-described construction of the prisms 2,the Fresnel lens sheet 1 constituting the rear projection screen 10 doesnot produce stray light, or even if it produces stray light, the amountof stray light produced is small, so that the image projected on therear projection screen 10 is entirely uniform in brightness and that theimage is never degraded by such troubles as the production of doubleimages.

Second Embodiment

Next, a Fresnel lens sheet according to the second embodiment of thepresent invention and a rear projection screen comprising it will bedescribed with reference to FIGS. 5 to 9. The Fresnel lens sheetaccording to the second embodiment of the present invention isequivalent to the Fresnel lens sheet according to the aforementionedfirst embodiment to which a low-refraction layer or light-diffusingelement is further provided on its emergent-side surface (plane ofemergence), and the other basic construction of the second embodiment isalmost the same as that of the first embodiment described above. In thefirst and second embodiments of the present invention, like referencenumerals are used to designate like parts, and detailed descriptions forthese parts are omitted.

(Arrangement of Fresnel Lens Sheet)

As shown in FIG. 5, in the Fresnel lens sheet 21 according to the secondembodiment of the invention, a coating layer (low-refraction layer) 22is formed on the plane of emergence 1 b of a base 1 a whoseincident-side surface is provided with prisms 2. The coating layer 22 isformed at least on a specific part of the plane of emergence 1 b of thebase 1 a, from which part imaging light LS emerges in the directioninclined toward the projector 5 side.

The coating layer 22 is made from a material whose refractive index islower than that of a material for forming the base 1 a. Specifically,fluorocarbon resins, silicone resins, and the like may be used asmaterials for forming the coating layer 22.

Thus, in the Fresnel lens sheet 21 shown in FIG. 5, the coating layer 22is formed on the plane of emergence 1 b of the base 1 a by the use of amaterial whose refractive index is lower than that of a material forforming the base 1 a, so that the following characteristic features areimpaired to the Fresnel lens sheet 21.

The characteristic features of the Fresnel lens sheet 21 shown in FIG. 5will be described hereinafter with reference to FIGS. 5 and 6. FIG. 6 isan illustration showing the behavior of imaging light LS in those prisms2 that are situated in an area of the Fresnel lens sheet 1 having nocoating layer on the plane of emergence 1 b of the base 1 a, on whicharea the imaging light LS is incident at a small angle θ (an area closeto the projector 5).

As shown in FIG. 6, in the Fresnel lens sheet 1 having no coating layeron the plane of emergence 1 b of the base 1 a, imaging light LSprojected on-each prism 2 is refracted at the plane of refraction 3 ofthe prism 2 and is then totally reflected at the plane of totalreflection 4 of the prism 2 in the direction inclined toward theprojector 5 side relative to the normal perpendicular to the sheetplane; the totally reflected light travels toward the plane of emergence1 b of the base 1 a. A part (e.g., approximately 4%) of the imaginglight LS that has arrived at the plane of emergence 1 b is reflected atthe plane of emergence 1 b and returns to the incident side. If such aFresnel lens sheet 1 is used together with a projection systemcomprising a mirror 6 as is shown in FIG. 9, the imaging light LSreflected from the plane of emergence 1 b of the base 1 a of the Fresnellens sheet 1 toward the incident side is reflected at the mirror 6 inthe projection system; this reflected light passes again through theFresnel lens sheet 1 and emerges from the plane of emergence 1 b. Thelight emerging from the plane of emergence 1 b of the Fresnel lens sheet1 after following the above-described light path is not perceived as animage because the light path is considerably long. However, if thislight and the imaging light emerging as normal light from the same spotoverlap each other, image contrast is lowered.

On the contrary, as shown in FIG. 5, in the Fresnel lens sheet 21 havingthe coating layer 22 on the plane of emergence 1 b of the base 1 a,imaging light LS that has been refracted at the plane of refraction 3 ofeach prism 2 and then totally reflected at the plane of total reflection4 of the prism 2 in the direction inclined toward the projector 5 siderelative to the normal perpendicular to the sheet plane reaches thecoating layer 22 formed on the plane of emergence 1 b. Since the coatinglayer 22 is made from a material having a low refractive index, theimaging light LS that is reflected from the plane of emergence 1 btoward the incident side is very few (e.g., approximately 2%). For thisreason, the Fresnel lens sheet 21 shown in FIG. 5 can effectivelyprevent lowering of image contrast even when used, as shown in FIG. 9,along with a projection system comprising a mirror 6.

To prevent lowering of image contrast, although the coating layer 22 is,in the Fresnel lens sheet 21 shown in FIG. 5, formed on the plane ofemergence 1 b of the base 1 a, a rough surface (light-diffusing element)23 may be provided, instead of the coating layer 22, on the plane ofemergence 1 b of the base 1 a as in the Fresnel lens sheet 21′ shown inFIG. 7. The rough surface 23 is provided at least on a specific part ofthe plane of emergence 1 b of the base 1 a, from which part the imaginglight LS emerges in the direction inclined toward the projector 5 side.

As shown in FIG. 7, in the Fresnel lens sheet 21′ having the roughsurface 23 on the plane of emergence 1 b of the base 1 a, the imaginglight LS that has been refracted at the plane of refraction 3 of eachprism 2 and then totally reflected at the plane of total reflection 4 ofthe prism 2 in the direction inclined toward the projector 5 siderelative to the normal perpendicular to the sheet plane is partlyreflected at the rough surface 23 on the plane of emergence 1 b andreturns to the incident side, where the rough surface 23 scatters thelight when it reflects the light. Therefore, when such a Fresnel lenssheet 21′ is used together with a projection system containing a mirror6 as is shown in FIG. 9, and even if the light reflected from the planeof emergence 1 b of the base 1 a of the Fresnel lens sheet 21′ towardthe incident side is reflected at the mirror 6 in the projection systemand returns again to the Fresnel lens sheet 21′, the intensity of thisreflected light is fully decreased, and lowering of image contrast canthus be effectively prevented as in the case of the Fresnel lens sheet21 shown in FIG. 5.

Although the rough surface 23 is, in the above embodiment, provided onthe plane of emergence 1 b of the base 1 a of the Fresnel lens sheet21′, it may be provided not on the plane of emergence 1 b but on aspecific portion of the plane of refraction 3 of each prism 2 on theFresnel lens sheet 21′, on which portion the imaging light LS reflectedfrom the plane of emergence 1 b is incident. If the rough surface 23 isprovided on such a portion, the imaging light LS that has returned tothe incident side is partly scattered, so that lowering of imagecontrast can be effectively prevented as in the case of the Fresnel lenssheet 21′ in which the rough surface 23 is provided on the plane ofemergence 1 b of the base 1 a.

On the other hand, instead of the rough surface 23 that is provided onthe plane of emergence 1 b of the base 1 a of the Fresnel lens sheet21′, a lenticular lens (light-diffusing element) 24 may be provided onthe plane of emergence 1 b of the base 1 a as in the Fresnel lens sheet21″ shown in FIG. 8. The lenticular lens 24 is provided at least on aspecific part of the plane of emergence 1 b of the base 1 a, from whichpart the imaging light LS emerges in the direction inclined toward theprojector 5 side.

As shown FIG. 8, in the Fresnel lens sheet 21″ having the lenticularlens 24 on the plane of emergence 1 b of the base 1 a, the imaging lightLS that has been refracted at the plane of refraction 3 of each prism 2and then totally reflected at the plane of total reflection 4 of theprism 2 in the direction inclined toward the projector 5 side relativeto the normal perpendicular to the sheet plane is partly reflected atthe lenticular lens 24 on the plane of emergence 1 b and returns to theincident side, where the lenticular lens 24 diffuses the light when itreflects the light. Therefore, when such a Fresnel lens sheet 21″ isused together with a projection system comprising a mirror 6 as is shownin FIG. 9, and even if the light reflected from the plane of emergence 1b of the base 1 a of the Fresnel lens sheet 21″ toward the incident sideis reflected at the mirror 6 in the projection system and returns againto the Fresnel lens sheet 21″, the intensity of this reflected light isfully decreased, and lowering of image contrast can thus be effectivelyprevented as in the case of the Fresnel lens sheet 21 shown in FIG. 5.

It is enough for the Fresnel lens sheet 21 (21′, 21″) shown in FIG. 5, 7or 8 to have the coating layer 22, the rough surface 23, or thelenticular lens 24 at least on a specific part of the plane of emergence1 b of the base 1 a, from which part the imaging light LS emerges in thedirection inclined toward the projector 5 side, and the coating layer22, the rough surface 23, or the lenticular lens 24 may be provided onany part covering the above-described part (e.g., the entire part of thesheet plane).

(Process of Producing Fresnel Lens Sheet)

Next, a process of producing the Fresnel lens sheet 21 (21′, 21″)according to the aforementioned second embodiment will be described.

Firstly, to produce the Fresnel lens sheet 21 shown in FIG. 5, a Fresnellens sheet that is the same as the Fresnel lens sheet 1 is obtained bythe production process according to the above-described firstembodiment, and a material having a low refractive index, such as afluorocarbon or silicone resin, is applied to the plane of emergence 1 bof the base 1 a of this Fresnel lens sheet 1 by such a coating method asflow or dip coating to form a coating layer 22. The coating layer 22 maybe formed on the plane of emergence 1 b of the base 1 a not by the abovemethod but by vacuum deposition of multiple layers of inorganicmaterials such as MgF₂, SiO₂ and TiO₂ that are different in refractiveindex.

Further, in the case where an ultraviolet-curing resin method isemployed as a process of producing the Fresnel lens sheet 21′ or 22″shown in FIG. 7 or 8, a substrate having, on one surface, a roughsurface 23 or lenticular lens 24 is laminated to an ultraviolet-curingresin applied to a mold. On the other hand, when a casting orhot-pressing method is used for producing the Fresnel lens sheet 21′ or22″, a mold having a molding surface engraved with the shape of a roughsurface 23 or lenticular lens 24 may be used.

(Rear Projection Screen Comprising Fresnel Lens Sheet)

The Fresnel lens sheet 21 (21′, 21″) shown in FIG. 5, 7 or 8 canconstitute a rear projection screen by itself. Alternatively, as shownin FIG. 9, the Fresnel lens sheet 21 (21′, 21″) may constitute a rearprojection screen 30 together with a lenticular lens sheet(light-diffusing sheet) 11 that is placed on the viewer's side of theFresnel lens sheet 21 (21′, 21″).

In the case of the rear projection screen 30 shown in FIG. 9, aprojector 5 is placed slantingly below the screen 30 at its rear, and byallowing a mirror 6 to reflect imaging light LS projected from thisprojector 5, the imaging light LS is projected on the rear projectionscreen 30 and is displayed to viewers.

In this case, owing to the above-described arrangement of the prisms 2,the rear projection screen 30 comprising the Fresnel lens sheet 21 (21′,21″) does not produce stray light, or even if it produces stray light,the amount of stray light produced is small, so that the image projectedon the rear projection screen 30 is entirely uniform in brightness andthat the image is never degraded by such troubles as the production ofdouble images, as in the case of the rear projection screen 10 accordingto the aforementioned first embodiment.

Moreover, in the rear projection screen 30 comprising the Fresnel lenssheet 21, since a coating layer 22 is formed on the plane of emergence 1b of the base 1 a of the Fresnel lens sheet 21, the reflection ofimaging light LS that occurs at the plane of emergence 1 b of the base 1a is reduced, and an image with high contrast can thus be obtained.Similarly, in the rear projection screen 30 comprising the Fresnel lenssheet 21′ or 21″, since a rough surface 23 or lenticular lens 24 isprovided on the plane of emergence 1 b of the base 1 a of the Fresnellens sheet 21′ or 21″, the imaging light LS is scattered or diffusedwhen it is reflected at the plane of emergence 1 b of the base 1 a, andan image with high contrast can thus be obtained.

Third Embodiment

Next, a Fresnel lens sheet according to the third embodiment of thepresent invention and a rear projection screen comprising it will bedescribed with reference to FIGS. 10 and 11. The Fresnel lens sheetaccording to the third embodiment of the present invention is equivalentto the Fresnel lens sheet according to the aforementioned firstembodiment to which additional prisms are further provided on itsemergent-side surface (plane of emergence), and the other basicarrangement of this embodiment is almost the same as that of the firstembodiment described above. In the first and third embodiments of thepresent invention, like reference numerals are used to designate likeparts, and detailed descriptions for these parts are omitted.

(Arrangement of Fresnel Lens Sheet)

As shown in FIG. 10, in a Fresnel lens sheet 41 according to the thirdembodiment of the invention, a plurality of additional prisms 42 areformed on the plane of emergence 1 b of a base 1 a whose incident-sidesurface is provided with prisms 2. The additional prisms 42 are formedconcentrically around the point positioned at the same height as that atwhich the center P of the concentric circles of the prisms 2 exists.Moreover, the additional prisms 42 are formed at least on a specificpart of the plane of emergence 1 b of the base 1 a, from which partimaging light LS emerges in the direction inclined toward the projector5 side.

The additional prisms 42 are formed to have triangular cross-sections,and each additional prism 42 has a lens plane 43 by which the light thathas been refracted at the plane of refraction 3 of each prism 2 on theincident side and then totally reflected at the plane of totalreflection 4 of the prism 2 in the direction inclined toward theprojector 5 side is allowed to emerge in the direction of the normalperpendicular to the sheet plane; and a non-lens plane 44 that connectstwo neighboring lens planes 43.

Thus, in the Fresnel lens sheet 41 shown in FIG. 10, a plurality of theadditional prisms 42 are formed on the plane of emergence 1 b of thebase 1 a, so that, of the imaging light LS that has passed through theprisms 2, the imaging light LS inclined toward the projector 5 siderelative to the normal perpendicular to the sheet plane of the base 1 acan be adjusted to emerge nearly vertically to the sheet plane of thebase 1 a.

As described above, in the Fresnel lens sheet 41 shown in FIG. 10, theadditional prisms 42 and the base 1 a are formed integrally.Alternatively, the additional prisms 42 may be formed on a prism sheet45 that is prepared separately from the base 1 a, as in the Fresnel lenssheet 46 shown in FIG. 11. Namely, a prism sheet 45 having, on itsemergent-side surface, additional prisms 42 whose structure is the sameas that of the additional prisms in the Fresnel lens sheet 41 shown inFIG. 10 is prepared, and is then adhered to the plane of emergence 1 bof the base 1 a with a transparent adhesive agent 47 or the like,thereby making the Fresnel lens sheet 46. Even in such a Fresnel lenssheet 46, of the imaging light LS that has passed through the prisms 2,the imaging light LS inclined toward the projector 5 side relative tothe normal perpendicular to the sheet plane of the base 1 a can beadjusted by the additional prisms 42 on the prism sheet 45 to emergenearly vertically to the sheet plane of the base 1 a.

Although the additional prisms 42 are, in the Fresnel lens sheet 46shown in FIG. 11, formed on the emergent-side surface of the prism sheet45, they may be formed not on the emergent-side surface but on theincident-side surface of the prism sheet 45. It is, however, preferablefrom the viewpoint of reduction of loss of light that the additionalprisms 42 be formed on the emergent-side surface of the prism sheet 45,as shown in FIG. 11.

Further, although the additional prisms 42 are, in the Fresnel lenssheets 41 and 46 shown in FIGS. 10 and 11, formed concentrically aroundthe point positioned at the same height as that at which the center P ofthe concentric circles of the prisms 2 exists, they may also be formedlinearly (the edge of each prism extending in the horizontal direction)or concentrically on the sheet plane around the center of the sheetplane of the base 1 a (the geometric center such as the center ofgravity).

Furthermore, it is enough for the Fresnel lens sheet 45 or 46 shown inFIG. 10 or 11 to have the additional prisms 42 at least on a specificpart of the plane of emergence 1 b of the base 1 a or of theemergent-side surface of the prism sheet 45, from which part the imaginglight LS emerges in the direction inclined toward the projector 5 side.Therefore, the additional prisms 42 may be formed in any part coveringthe above-described part (e.g., the entire part of the sheet plane).

(Process of Producing Fresnel Lens Sheet)

Next, a process of producing the Fresnel lens sheets 41 and 46 accordingto the aforementioned third embodiment will be described.

Firstly, to produce the Fresnel lens sheet 41 shown in FIG. 10, aFresnel lens sheet that is the same as the Fresnel lens sheet 1 isobtained by the production process according to the above-describedfirst embodiment; a mold is laid, together with an ultraviolet-curingresin, on the emergent-side surface of this Fresnel lens sheet; andultraviolet light is then applied to cure the ultraviolet-curing resinin the mold.

On the other hand, in producing the Fresnel lens sheet 46 shown in FIG.11, a prism sheet 45 having additional prisms 42 on its emergent-sidesurface is prepared by the production process according to the firstembodiment described above. Specifically, using a cutting tool or thelike, a mold is firstly prepared by engraving a mold material with theshape corresponding to the incident-side surface of the prism sheet 45,and an ultraviolet-curing resin is applied to this mold. On thisultraviolet-curing resin applied to the mold, a substantiallytransparent substrate that transmits ultraviolet light is laid, andpressure is applied to the substrate by a pressure roller or the like,thereby bringing the substrate into close contact with theultraviolet-curing resin. Thereafter, from above the substrate laminatedin this manner, ultraviolet light is applied to cure theultraviolet-curing resin. The ultraviolet-curing resin cured is thenreleased from the mold and cut into the desired size, thereby obtainingthe Fresnel lens sheet 45.

Thereafter, with a transparent adhesive agent 47 or the like, the prismsheet 45 produced in the above-described manner is adhered to the planeof emergence 1 b of the base 1 a of a Fresnel lens sheet, the same asthe Fresnel lens sheet 1, produced by the production process accordingto the aforementioned first embodiment. Thus, there is obtained theFresnel lens sheet 46.

(Rear Projection Screen Comprising Fresnel Lens Sheet)

The Fresnel lens sheet 41 or 46 shown in FIG. 10 or 11 can constitute arear projection screen by itself. Alternatively, as in theabove-described first and second embodiments, the Fresnel lens sheet 41or 46 may constitute a rear projection screen together with a lenticularlens sheet (light-diffusing sheet) that is placed on the viewer's sideof the Fresnel lens sheet 41 or 46.

In this case, owing to the above-described arrangement of the prisms 2,the rear projection screen comprising the Fresnel lens sheet 41 or 46does not produce stray light, or even if it produces stray light, theamount of stray light produced is small, so that the image projected onthe rear projection screen is entirely uniform in brightness and thatthe image is never degraded by such troubles as the production of doubleimages, as in the cases of the rear projection screens 10 according tothe first and second embodiments described above.

Moreover, in the rear projection screen comprising the Fresnel lenssheet 41 or 46, the additional prisms 42 are formed on the plane ofemergence 1 b of the base 1 a or on the emergent-side surface of theprism sheet 45, so that, of the imaging light LS that has passed throughthe prisms 2, the imaging light LS inclined in the direction toward theprojector 5 side relative to the normal perpendicular to the sheet planeof the base 1 a emerges nearly vertically to the sheet plane of the base1 a. Therefore, the rear projection screen can display a bright image toviewers.

EXAMPLES Example 1

A rear projection screen with a screen size of 50 inches (4:3) wasassembled, as a rear projection screen of Example 1, from the Fresnellens sheet and the lenticular lens sheet described under the following(1) and (2), respectively. A projector was placed at the center, in thehorizontal direction, of the rear projection screen, but 312 mm belowthe lower edge of the rear projection screen and 400 mm distant from therear projection screen in the direction normal to it, and imaging lightwas projected on the rear projection screen from this projector. Therear projection screen of Example 1 corresponds to the rear projectionscreen according to the first embodiment described above.

(1) Fresnel Lens Sheet

A Fresnel lens sheet having, on the plane of incidence of a base insheet form, a plurality of prisms in the shape of circular arcs, eachprism having a plane of refraction and a plane of total reflection, wasprepared as a Fresnel lens sheet of Example 1. These prisms were formedon a polycarbonate substrate (refractive index 1.58) by applying theretoan ultraviolet-curing resin, followed by shaping. The prisms wereconcentrically formed around the point positioned at the center, in thehorizontal direction, of the rear projection screen, but 312 mm belowthe lower edge of the rear projection screen. The pitch of the prismswas made 0.11 mm. Therefore, the radii of the prisms in the shape ofcircular arcs became between 312 mm and 1188 mm. Further, the prismswere formed to have an apical angle of 38°, and a material having arefractive index of 1.55 was used for forming the prisms. Regarding theangle of incidence at which the imaging light is projected on the rearprojection screen, the angle at which the imaging light was incident onthe prism with a radius of 312 mm, the minimum angle of incidence, wasmade 38°, while the angle at which the imaging light was incident on theprism with a radius of 1188 mm, the maximum angle of incidence, was made71.4°.

The prisms were made so that the imaging light that had passed throughthe prism with a radius of 312 mm emerged in the direction inclineddownward (toward the projector side) at an angle of 10° and that theangle of inclination (the angle between the direction in which theimaging light emerges and the normal perpendicular to the sheet plane)was gradually decreased to such an extent that the imaging light thathad passed through the prism with a radius of 700 mm emerged verticallyto the sheet plane (angle of inclination relative to the verticaldirection: 0°). The prisms with radii of 700 to 1188 mm were made sothat the imaging light that had passed through these prisms emergedvertically to the sheet plane (angle of inclination relative to thevertical direction: 0°). FIG. 12 is a diagram showing the relationshipbetween the radius of the prism and the angle of inclination of theimaging light in Example 1.

(2) Lenticular Lens Sheet

A lenticular lens sheet that had linear lenses with half-ellipticcross-sections, extending in the vertical direction (direction of up anddown) on the plane of incidence at a pitch of 0.14 mm, was prepared as alenticular lens sheet of Example 1. The thickness of the lenticular lenssheet was made 1 mm, and a diffusing agent was dispersed in thelenticular lens sheet, thereby making the half angle of horizontaldiffusion 30° and the half angle of vertical diffusion 12°. Further, alight-absorbing layer with a thickness of 20 μm was formed on thesurfaces of the lenses with half-elliptic cross-sections. The absorbanceof the light-absorbing layer was made 40%.

A rear projection screen was assembled from the above-described Fresnellens sheet and lenticular lens sheet. Imaging light was projected onthis rear projection screen from the above-described projector, and theimage displayed on the rear projection screen was observed. As a result,no degradation caused by such troubles as the production of doubleimages occurring due to stray light was found in the image, and theimage was thus confirmed to be excellent.

Example 2

A rear projection screen with a screen size of 50 inches (4:3) wasassembled, as a rear projection screen of Example 2, from the Fresnellens sheet and the lenticular lens sheet described under the following(1) and (2), respectively. Directing to the side opposite to theincident side of the rear projection screen, a projector was placed atthe center, in the horizontal direction, of the rear projection screen,but 312 mm below the lower edge of the rear projection screen and 100 mmdistant from the rear projection screen in the direction normal to it,and a mirror was also placed at the center, in the horizontal direction,of the rear projection screen, but 200 mm below the lower edge of therear projection screen and 250 mm distant from the rear projectionscreen in the direction normal to it, whereby the mirror was allowed toreflect, toward the rear projection screen, the imaging light projectedfrom the projector. The rear projection screen of Example 2 correspondsto the rear projection screen according to the above-described secondembodiment, comprising a coating layer on the emergent-side surface ofthe Fresnel lens sheet.

(1) Fresnel Lens Sheet

A Fresnel lens sheet having the same arrangement as that of the Fresnellens sheet of Example 1 but comprising a coating layer formed on theemergent-side surface of the Fresnel lens sheet was used as a Fresnellens sheet of Example 2. The coating layer was formed by laminating a250 nm thick TiO₂ layer (refractive index 2.4) to the polycarbonatesubstrate (refractive index 1.58) constituting the Fresnel lens sheetand a 120 nm thick SiO₂ layer (refractive index 1.46) to this TiO₂layer. The reflectance of the coating layer was made 1% (in the case ofvertical incidence).

(2) Lenticular Lens Sheet

The same lenticular lens sheet as that of Example 1 was used as alenticular lens sheet of Example 2.

A rear projection screen was assembled from the above-described Fresnellens sheet and lenticular lens sheet. Imaging light was projected onthis rear projection screen from the above-described projection system,and the image displayed on the rear projection screen was observed. As aresult, neither degradation caused by such troubles as the production ofdouble images occurring due to stray light nor lowering of contrast wasfound in the image, and the image was thus confirmed to be excellent.

Example 3

A rear projection screen with a screen size of 50 inches (4:3) wasassembled, as a rear projection screen of Example 3, from the Fresnellens sheet and the lenticular lens sheet described under the following(1) and (2), respectively. Directing to the side opposite to theincident side of the rear projection screen, a projector was placed atthe center, in the horizontal direction, of the rear projection screen,but 312 mm below the lower edge of the rear projection screen and 100 mmdistant from the rear projection screen in the direction normal to it,and a mirror was also placed at the center, in the horizontal direction,of the rear projection screen, but 200 mm below the lower edge of therear projection screen and 250 mm distant from the rear projectionscreen in the direction normal to it, whereby the mirror was allowed toreflect, toward the rear projection screen, imaging light projected fromthe projector. The rear projection screen of Example 3 corresponds tothe rear projection screen according to the above-described secondembodiment, comprising a lenticular lens (light-diffusing element) onthe emergent-side surface of the Fresnel lens sheet.

(1) Fresnel Lens Sheet

A Fresnel lens sheet having the same arrangement as that of the Fresnellens sheet of Example 1 but comprising lenticular lenses extending inthe horizontal direction, formed on the emergent-side surface of theFresnel lens sheet, was used as a Fresnel lens sheet of Example 3. Thelenticular lenses were formed to have half-circular cross-sections witha radius of 0.1 mm, and the pitch of the lenticular lenses was made 0.08mm.

(2) Lenticular Lens Sheet

The same lenticular lens sheet as that of Example 1 was used as alenticular lens sheet of Example 3.

A rear projection screen was assembled from the above-described Fresnellens sheet and lenticular lens sheet. Imaging light was projected onthis rear projection screen from the above-described projection system,and the image displayed on the rear projection screen was observed. As aresult, neither degradation caused by such troubles as the production ofdouble images occurring due to stray light nor lowering of contrast wasfound in the image, and the image was thus confirmed to be excellent.

1. A Fresnel lens sheet useful for condensing imaging light obliquelyprojected from a projector to let the light emerge toward a viewer'sside as nearly parallel rays, comprising: a base in sheet form; and aplurality of prisms formed on an incident side of the base, each of theprisms having a plane of refraction that refracts the imaging lightprojected and a plane of total reflection that totally reflects, towardthe viewer's side, at least a part of the light refracted at this planeof refraction, wherein at least some of the plurality of prisms are somade that the imaging light that has been incident at an angle of notmore than 40° and has passed through the Fresnel lens sheet emerges in adirection inclined toward a projector side at a predetermined angle ofnot more than 13° with respect to a normal perpendicular to a sheetplane of the base.
 2. The Fresnel lens sheet according to claim 1,wherein the at least some of the plurality of prisms are those prismsthat are situated in an area on which the imaging light projected fromthe projector is incident at a small angle to become stray light.
 3. TheFresnel lens sheet according to claim 1, wherein the plurality of prismsare so made that the angle at which the imaging light that has passedthrough the prisms is inclined increases gradually as an angle at whichthe imaging light is incident on the prisms decreases.
 4. The Fresnellens sheet according to claim 1, further comprising, at least on aspecific part of an emergent-side surface of the base, from which partthe imaging light emerges in the direction inclined toward the projectorside, a low-refraction layer made from a material whose refractive indexis lower than that of a material for forming the base.
 5. The Fresnellens sheet according to claim 1, further comprising a light-diffusingelement provided at least on a specific part of an emergent-side surfaceof the base, from which part the imaging light emerges in the directioninclined toward the projector side.
 6. The Fresnel lens sheet accordingto claim 5, wherein the light-diffusing element is a rough surfaceprovided on the emergent side of the base.
 7. The Fresnel lens sheetaccording to claim 5, wherein the light-diffusing element is alenticular lens provided on the emergent side of the base.
 8. TheFresnel lens sheet according to claim 1, further comprising, on anemergent side of the base, a plurality of additional prisms by which, ofthe imaging light that has passed through the prisms, the imaging lightinclined toward the projector side relative to the normal perpendicularto the sheet plane of the base is adjusted to emerge nearly verticallyto the sheet plane of the base.
 9. A rear projection screen comprising:a Fresnel lens sheet according to claim 1; and a light-diffusing sheetplaced on the viewer's side of the Fresnel lens sheet.